I tried this plugin on a test installation.
Once I had activated it, I tried out the configurations. One of them replaces every already-assigned featured image with a category image instead. I did not like that effect, and wanted to re-feature my original choices. I switched it off-- or so I thought.
But I found that was no longer able to assign featured images to posts that I edited or made new. The field was no longer available on post-new.php. It does not show up as in the screen options at the top of the page, either.
I deactivated the plugin, but the featured image field still did not show up. I had to reinstall wordpress.